This invention relates to a cathode for carbonate fuel cells and, in particular, to a ribbed cathode for such cells.
Carbonate fuel cells produce energy in a clean and efficient way by directly converting chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell unit comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In carbonate fuel cells, reactant fuel gas is passed through the anode, while oxidizing gas is passed through the cathode. In order to produce a useful power level, a number of individual fuel cell units are stacked in series with an electrically conductive separator plate between each cell.
A conventional carbonate fuel cell cathode assembly incorporates a stainless steel cathode current collector that forms oxidant gas channels and conducts electricity from the cathode to a bipolar plate. The cathode of the assembly, in turn, is flat or planar, and is generally prepared by dry doctoring and sintering filamentary nickel powder. The cathode is also typically filled with electrolyte before assembling it into the fuel cell.
The conventional cathode assembly results in a large amount of electrolyte loss from the fuel cell. It has been experimentally determined that approximately 25% of the lithium carbonate in the electrolyte is depleted after 40,000 hours of operation due to corrosion of the current collector of the assembly. In particular, the current collector is responsible for approximately 70% of the total electrolyte loss.
Because fuel cells operate under a constant compressive load, another difficulty experienced with the conventional fuel cell cathode assembly is creep of the cathode. Cathode creep is the susceptibility of the cathode to shrink and become compact under the constant compressive forces in the fuel cell. Cathode creep is inversely proportional to the cathode porosity, where the susceptibility of the cathode to shrink and become compact increases with higher porosity. At the same time, the electrochemical performance and electrolyte storage capacity of the cathode are directly proportional to the cathode porosity. Therefore, the cathode porosity is determined by a compromise between minimizing shrinkage and maximizing performance and electrolyte storage capacity.
Another type of cathode assembly utilizes a ribbed cathode instead of the flat or planar cathode. The ribbed cathode itself provides the required gas flow channels and thus the cathode assembly does not require a current collector to deliver the gas to the cathode. In a carbonate fuel cell system, a ribbed cathode may provide some advantages over the baseline or conventional flat cathode-current collector cathode assembly, including increased cell inventory of the electrolyte, reduction in the loss of the electrolyte due to corrosion of the current collector and the removal of the costs associated with fabricating the cathode current collector. In spite of these advantages, the increased thickness of the ribbed cathode makes it more susceptible to cathode creep.
Several patents disclose methods of fabricating ribbed electrodes, including anodes and cathodes. For example, U.S. Pat. No. 4,654,195 describes a method for fabricating a ribbed anode with strengthening additives in which the electrode is formed and pre-fired in a mold. The formed electrode is then removed from the mold and sintered in a reducing atmosphere. The electrode may then be optionally compacted to achieve a desired final thickness. U.S. Pat. No. 5,531,956 describes another method of producing a ribbed electrode that includes the steps of depositing a suspension of a powdered electrode metal onto the face of a substantially flat porous electrode metal substrate, forming raised structures on the face of the electrode, and then sintering the electrode.
As above-mentioned, the ribbed cathodes formed by these procedures still exhibit significant creep in the absence of strengthening additives.
It is therefore an object of the present invention to provide a ribbed cathode possessing excellent strength and electrochemical performance characteristics, while minimizing cathode creep.
It is a further object of the present invention to provide a ribbed cathode adapted to provide an increase fuel cell life, particularly for carbonate fuel cells, by reducing electrolyte loss during operation and increasing electrolyte storage capacity.
It is also an object of the present invention to provide a ribbed cathode adapted to permit an increase in the lithium concentration of the fuel cell electrolyte, thereby reducing electrolyte evaporation and cathode dissolution rates.